1. Field of the Invention
Embodiments of the invention are directed to individual aspheric intra ocular lenses (IOLs) including multi-component accommodating intraocular lenses (referred to herein as “A-IOLs”) that provide specialized control of spherical aberration and other physical lens parameters; to a family of aspheric intraocular lenses including a family of multi-component accommodating intraocular lenses having consistent labeling, selection and performance parameters; and to a method for designing such IOLs, A-IOLs and associated lens families.
2. Description of Related Art
A simple optical system consists of a lens, which can form an image of an object. In the most basic, ideal situation, a perfect plane wavefront coming from an object located an infinite distance from the lens will be imaged to a focal point one focal length away from the lens along an optical axis of the optical system. Lens defects induce aberrations to the wavefronts of light from an object as they pass through the lens resulting in an image that is blurry.
Different types of lens defects or optical system defects produce different types and degrees of aberrations that may generally appear similar to the naked eye. For example, if a perfect lens is moved along the optical axis of the optical system, the image of the object formed by the lens will suffer from defocus. Stated differently, if the surface upon which the image is viewed is moved along the optical axis, the image will likewise be defocused. The aberration of astigmatism results from in an optical system having a different focusing power in the horizontal direction than in the vertical direction, for example, resulting in a distorted image at every image location. Another troublesome aberration known as spherical aberration, illustrated in FIG. 1, is produced by a lens 5 having spherical surfaces 11, 12. Light ray bundle 7 passing through the lens near its center is brought to a focus at a different position on the optical axis than the light ray bundles 6, 8 passing through the lens nearer its circumference. By convention, the spherical aberration of a lens is measured by the longitudinal or transverse distance between the center- and edge-focused rays of light incident on the lens as a plane wavefront originating at an optically infinite object distance, O. This is referred to as inherent spherical aberration. If a spherical lens, which by definition has inherent spherical aberration, is decentered with respect to the optical axis passing through the center of the lens, then the resulting image will be affected by other aberrations including coma and astigmatism. As mentioned above, any one or combination of these aberrations will cause the image to appear blurry, washed out or otherwise lacking in subjective quality.
The optical system of the eye is known as an ocular system, illustrated in FIG. 2. In simple anatomical terms, the ocular system 100 is comprised of the cornea 1, the iris 2, the crystalline lens 3, and the retina 4. The cornea is the first component of the ocular system to receive light coming from an object and provides roughly two-thirds of the principal focusing capability of the ocular system. The crystalline lens provides the remaining focusing capability of the eye. If a plane wavefront coming from an object located at optical infinity is focused by the cornea and crystalline lens to a point in front of the retina, the eye is referred to as myopic. On the other hand, if the combined focusing power of the cornea and crystalline lens is too weak such that a plane wavefront is focused behind the retina, the ocular system is referred to as hyperopic. The function of the iris is to limit the amount of light passing through the ocular system. The crystalline lens is uniquely adapted to fine tune the focusing ability of the ocular system allowing the healthy eye to form sharp images of objects both far away and up close. The retina is the image detector of the ocular system and the interface between the eye and the brain.
As people age, the crystalline lens loses its capability to allow the ocular system to form images on the retina of near objects (i.e., closer than about 10 inches). This phenomenon is known as presbyopia. Presbyopia is the inability to accommodate or focus on an object close to the eye. In certain cases, an intraocular lens that is designed to restore the accommodative capability of the eye may be provided. These lenses are referred to as accommodating intraocular lenses (accommodating IOLs). These accommodating IOLs may be of a single optic design or a multi-component (typically two-optic) design referred to herein as a multi-component accommodating IOL (A-IOL). Although accommodating IOLs and A-IOLs have both shared and unique advantages, A-IOLs are considered to be able to provide a greater amount of accommodating power than their single element counterparts. Examples of alternative A-IOL designs are disclosed in U.S. Pat. Nos. 5,275,623; 6,423,094; 6,488,708; 6,858,040; and U.S. Published Application Nos. 2004/0015236 and 2003/0130732, the disclosures of which are incorporated by reference in their entireties to the fullest extent allowed by applicable laws and rules. Other complications, e.g., cataracts, may require that the defective crystalline lens be removed from the ocular system and a synthetic lens referred to as a pseudophakic intraocular lens (IOL) be put in its place. Alternatively, a phakic IOL may be implanted without removing the natural crystalline lens to correct refractive errors such as those correctable by spectacles, contact lenses or corneal refractive procedures (e.g. LASIK, CK, PRK, LASEK, etc.).
Although IOLs have been around for more than 40 years, they still do not provide the ocular system with the visual performance obtained with a healthy natural crystalline lens. This is partly due to material considerations, optical characteristics, placement accuracy and stability and other factors relating to the IOL that detract from optimal visual performance. In addition, the natural crystalline lens has certain aberrations of opposite sign to those same aberrations produced by the cornea, such that the total aberrations are reduced. This has been referred to as aberration emmetropization. In recognition of these factors, various solutions have been developed. For example, silicone has become a favored IOL material, in addition to PMMA, hydrogel, and hydrophilic and hydrophobic acrylic materials. Scores of haptic designs have been and continue to be developed to address the positioning and stability concerns of implanted IOLs. Accommodating IOLs and A-IOLs suffer from the same issues of positioning, stability and misalignment. Different surface shapes of IOLs have been provided to minimize lens weight and thickness and to control aberrations that degrade image quality. For illustration, Table 1 (Tables 1-4 are located at the end of the specification) lists the optical prescription and technical specifications of two exemplary IOLs referred to as: the LI61U, a conventional IOL with spherical anterior and posterior surfaces, manufactured by Bausch & Lomb Incorporated, Rochester, N.Y., and the Tecnis Z9000, an advanced IOL with a prolate anterior surface and a spherical posterior surface (Advanced Medical Optics, Santa Ana, Calif.). In brief, the LI61U lens has positive inherent spherical aberration as with any IOL having spherical surfaces. The Tecnis Z9000 IOL has negative spherical aberration in an amount designed to offset or counter balance the positive spherical aberration of the average cornea. While both of these lenses offer certain advantages, the Tecnis Z9000 lens is directed at controlling some component of spherical aberration in the ocular system to achieve improved image quality. The intended result thus appears as one of minimizing residual spherical aberration in the image for the average population. It is well known, however, that non-accommodating IOLs, accommodating IOLs and A-IOLs are subject to movement and resulting misalignment or decentering after implantation and, that, when a lens with spherical aberration is decentered, asymmetrical aberrations such as coma and astigmatism are introduced into the image. While the effects of spherical aberration can be effectively but not completely mitigated by spectacles, the effects of coma cannot.
In view of the foregoing, the inventor has recognized the need for IOLs accommodating IOLs and A-IOLs of alternative designs and construction that can selectively control spherical aberration, and which provide improved visual performance in ocular systems to a degree not provided by currently available lenses when used in these systems.
The availability of IOLs having different values of spherical aberration raises additional issues not heretofore dealt with in the art. Persons skilled in the art understand that an IOL is described and generally labeled for selection by two parameters: lens power and a lens constant such as, e.g., the A-constant (other lens constants may be referred to, for example, as a surgeon factor or ACD constant). A-IOLs may be similarly labeled with lens power and a lens constant, however the lens constant may differ from the typical A-constant used with IOLs and may work with a modified lens power formula. Labeled lens power is expressed as the paraxial power of the lens. The paraxial power of the lens is the power of the lens through the center region of the lens very close to the optical axis. A lens having inherent spherical aberration, however, has a true power that is different than the paraxial power of the lens. For example, in a spherical lens having positive spherical aberration, the power of the lens increases as a function of radial distance away from the center of the lens. For example, using the lens prescription data for the LI61U lens from Table 3 below, the radial profile of local power and average power is as follows:
Ray HeightLocal Power (D)DiameterAverage Power (D)022.00022.000.522.051.022.021.022.192.022.091.522.433.022.212.022.794.022.382.523.275.022.613.023.916.022.90Although this variation in power is generally, albeit imperfectly, accounted for by the various selection formulae used by surgeons for equiconvex spherical lens products, the standard formulae do not accurately account for the power variations in aspheric IOLs having inherent spherical aberration with different radial profiles.
An additional, practical concern is addressed in the following exemplary scenario. It is not uncommon for a surgeon who regularly performs IOL procedures to consistently use a limited number of IOL types or brands in their practice. For example, assume the surgeon generally prescribes the Tecnis Z9000 lens listed in Table 1 and the LI61U lens as his common alternative IOL. Each of these lens brands carries a different labeled lens A-constant (e.g., AZ9000=119; ALI61U=118). Using the standard lens power equation (P=A−2.5L−0.9K, where P is the power of the IOL to be implanted, A is the A-constant of the IOL, L is the measured axial length of the eye and K is the keratometric power of the cornea; see below) for selecting the appropriate IOL power would indicate the use of the Tecnis Z9000 lens having a paraxial power of 23D (and inherent negative spherical aberration), or the LI61U lens having a paraxial power of 22D (and inherent positive spherical aberration). Stated differently, because these lenses will have the same shape factor to account for their spherical aberration values; i.e., they are both equiconvex), they will be labeled as having different A-constants despite both of them having a power equal to 22D. Unless the surgeon (or more typically an assistant) correctly modifies the entry of data to account for the different A constant values of the two lenses, the patient risks having an IOL implanted whose power correction is off by one diopter. Not only is the patient's resulting vision sub-optimal, but there may be additional time, effort and inconvenience for the physician.
Accordingly, as different lenses, lens families and lens brands (including those now having different spherical aberration amounts) are available for selection by the surgeon, lenses having consistently labeled parameters that inform the surgeon of the desired, correct selection would be advantageous. The obvious advantages are the removal of guesswork on the part of the surgeon and removal of the need for the surgeon to invent new formulae to account for characteristics of the lens that may vary, such as true power and spherical aberration value. Another advantageous benefit will be realized by the lens manufacturer and pertains to various governmental approval processes for regulated products such as IOLs. For example, the approval from the US-FDA for a child-IOL having a labeled power and A-constant consistent with a parent-IOL in the exemplary case of the parent-IOL and the child-IOL having different spherical aberration values, will be considerably less burdensome and expensive than if the labeled parameters for the parent-IOL and child-IOL are necessarily different. (The term “parent-IOL” as used herein refers to an existing spherical lens or lens line identified by a labeled power and lens constant; the term “child-IOL” refers to a subsequent aspheric lens or lens line that is (or can be) labeled with the same lens power and lens constant as the parent lenses). Thus, there is a need for a family of IOLs whose individual members have characteristics that allow consistent, selection-based labeling of the lens products.
The inventor has also recognized that standardization of certain physical characteristics of A-IOLs would be advantageous both in terms of evaluating lens performance and for handling and inserting the A-IOL. For instance, if the anterior lens of a two-lens A-IOL has a substantially constant positive optical power over a broad power range for the A-IOL family, the posterior lens shape will necessarily change in order to vary the overall power of the A-IOL. For a certain range of negative optical powers, one or more physical parameters of the posterior lens (and thus the A-IOL) may become undesirable. For example, center thickness may become too thin for lens integrity, edge thickness may become too thick for a particular injector bore, lens volume, cross sectional area, thickness profile and/or shape may cause stability, insertion or other problems. Accordingly, a degree of constancy or standardization of one or more of these parameters over a family of A-IOLs will provide improvements in efficiency, cost and performance.